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ZingPath: Classification

Introduction to Protists

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Introduction to Protists


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You will learn about different types of protists by using a dichotomous key.

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Now You Know

After completing this tutorial, you will be able to complete the following:

  • List characteristics of the Kingdom Protista.
  • Identify the means of locomotion of specific protists.
  • Differentiate among protists based on their characteristics.
  • Identify given protista samples by examining them according to their nutrition and digestion styles, and whether they have organs that enable movement.

Everything You'll Have Covered

Protista is the most diverse eukaryotic kingdom, with members differing in their mode of nutrition and cell structure. The Kingdom Protista is defined by exclusion: Protists are eukaryotes that are not plants, animals, or fungi. Because evolutionary relationships among protists are unclear, their classification is based on a broad set of characteristics. The six major groups are: plant-like protists (algae), fungi-like protists (slime molds), and four groups of heterotrophic, animal-like protists-amoebas, ciliates, flagellates, and sporozoans.

Algae are classified as being unicellular or multicellular, as in seaweed and kelp. Some single-celled algae are mobile by means of flagella. However, the defining characteristic of this group is its ability to photosynthesize. This can be determined by incubating protists in a non-nutrient solution and comparing their numbers over time. If the population increases, they must be autotrophic. Slime molds, like fungi, carry out extracellular digestion. They tend to be decomposers and feed on dead organisms. Extracellular digestion can be detected by incubating the protist in a starch solution. After some time, the solution is subjected to a Benedict's reagent test for the presence of glucose. A positive test result indicates that the starch was digested into glucose outside the cell, and the Benedict's reagent solution turns from blue to brick red.

The animal-like protists are heterotrophic and unicellular. These include the aquatic amoeba, which moves by extending projections called pseudopods, formed via cytoplasmic streaming, and pulling itself forward. Other mobile protists are the ciliates, whose cell surfaces are covered with tiny hair-like projections called cilia that beat in a coordinated way to move the organism; and flagellates, which have one or more long projections called flagella, that whip back and forth to propel the cell. (Note that the eukaryotic flagellum differs in structure and evolutionary origin from the bacterial flagellum.) The last group, sporozoans, do not move; these organisms release spores for reproduction and tend to be parasitic.

Tutorial Details

Approximate Time 20 Minutes
Pre-requisite Concepts Students should be able to define the concept of classification, list the different kingdoms, and be able to describe the general properties of protists.
Course Biology
Type of Tutorial Concept Development
Key Vocabulary algae, amoeba, ciliate